A steam generator is a closed, heated vessel or a pressure pipe system used for generating high-pressure and high-temperature steam for heating and industrial purposes (e.g. for operating a steam turbine). If steam outputs and pressures are particularly high, as in energy generation in power plants, for example, water tube boilers are used in which the flow medium—usually water—flows in steam generator tubes. Water tube boilers are also used in solids combustion, as the combustion chamber in which heat is generated by combustion of the respective raw material can be freely configured by the arrangement of tube walls.
Such a steam generator designed as a water tube boiler therefore comprises a combustion chamber whose enclosing wall consists at least partly of tube walls, i.e. steam generator tubes welded together in a gas-tight manner. On the flow medium side, these steam generator tubes, as evaporator heating surfaces, first form an evaporator into which unevaporated medium is introduced and evaporated. The evaporator is usually disposed in the hottest part of the combustion chamber. Possibly provided downstream thereof on the flow medium side is a device for separating water and steam and a superheater in which the steam is further heated beyond its evaporation temperature in order to achieve a high degree of efficiency in a following heat engine such as a steam turbine, for example. Upstream of the evaporator on the flow medium side, a pre-heater (a so-called economizer) is provided which pre-heats feedwater using waste or residual heat, thereby likewise increasing the efficiency of the system as a whole.
Bare tubes or internally finned tubes are used for evaporator heating surfaces in steam generators. Internally finned tubes are used if the flow of the flow medium into the steam generator tubes is to be swirled, which produces a higher velocity of the fluid on the inner surface of the steam generator tubes. The use of said internally finned tubes may be required for various reasons, such as in the case of a low mass flux of the evaporator under full load. Internally finned tubes also need to be used in the case of high heat fluxes (e.g. in drum boilers): here there is a risk of film boiling, i.e. a film of steam forms on the inside of the steam generator tubes which produces a thermally insulating effect in contrast to the well mixed liquid in the case of nucleate boiling. As a consequence, with the heat flux remaining the same, the wall temperature may increase strongly which can result in destruction of the heating surfaces. Not least, internally finned tubes can prevent flow stratification under normal load conditions (e.g. at minimum load in spiral tube steam generators).
The internal finning of the tubes is produced according to the prior art in a cold drawing process. According to the current state of knowledge, internally finned tubes can only be made from materials having a maximum chromium content of 5%. If internally finned tubes made from steels having a higher chromium content are required to be used, e.g. due to the steam parameters being heightened still further to increase the efficiency, the internally finned tubes cannot be produced using the processes currently available. In this case bare tubes can be provided separately with appropriate swirl-generating inserts. These inserts can be produced independently of the bare tubes and can therefore also be made from a different material.